Nov for same reason above, am currently transitioning and birthday in july, 1 year transitioning day in august, but wont be straightening till december- jan. Its just not worth it in summer ( i mean hair reverts quickly).
I'ts your choice. The best time, IMO is whenever the idea makes you feel the best. In the meantime, you can always WOW people with your existing beautiful curls. I haven't straightened my hair yet except with rollersets.